Overhead projectors are devices which produce magnified images on a screen or wall of indicia-bearing transparencies, typically clear plastic sheets including textual and/or graphical information. The projector includes a light source and a lens system which directs the light through the transparency to the screen for viewing by a group. The light source is typically a high-intensity lamp. These lamps generate significant heat and are thus relatively short lived.
In order to avoid a long delay in a presentation because of lamp failure, overhead projectors are typically supplied with a mechanism which includes two or more lamps, one of which is positioned for use and the other of which is retained as a spare. When the lamp in use fails, the user need only operate a knob or similar actuating device to remove the failed lamp from the use position and insert the spare lamp in its place.
Although lamp changing mechanisms have been designed which operated successfully, these mechanisms have typically been unduly complex and did not always achieve proper alignment of the lamps with the lens system of the projector.